1. Field of the Invention
The present invention relates to monolithic microwave integrated circuits (MMIC's) and more particularly to the biasing of the radio frequency (RF) device of a microwave power amplifier.
2. Background Information
The recent increased demand for wireless communication fueled an increased demand for high performance RF power amplifier modules (PAM's), including RF power modules using MMIC's. These PAM's include a power amplifier chip and an external voltage regulator circuit that biases the power amplifier chip. The voltage regulator is designed to remove the PAM's sensitivity to variation in the DC supply Typically, the DC supply is a battery with substantial voltage variations during charging/discharging cycles.
Separate PAM's and regulator modules are expensive, complex, less reliable, and take up scarce space in the final assembly. However, traditional bipolar and/or CMOS circuitry used in regulator modules is not suitable for integration with the power amplifier due to operating issues, e.g. temperature.
FIG. 1 shows a typical unregulated mirror current circuit and the variation of the output current, Icc, with Vcc (the DC supply voltage) and temperature. In this circuit Icc is a mirror of Im. As can be seen from the graph, Icc increases by about ⅓ as temperature increases from about 0 to 80 degrees C., and there is a corresponding increase of Icc as the DC voltage increases. Typically, a battery will demonstrate as much as a +/−10% or more variation during a charging/discharging cycle. The result is, with temperature and battery voltage variations, Icc may nearly double.
Q1 and Q2 of FIG. 1 are current mirrors and if Im is held about constant, over temperature and Vcc variations, Icc will also be constant. The temperature variations, as known in the art, will be the temperature sensitivity of the base/emitter voltage of Q2 diminished by its relative value compared to the voltage drop across R1. FIG. 1 has no mechanism to hold Im constant with changing Vcc. Q1 and Q2, in this circuit, are GaAs HBT's (gallium arsenide hetro-junction bipolar transistors). The term HBT is defined herein to include the GaAs HBT. These transistors are very temperature and bias sensitive devices, typically affecting output signal linearity. Silicon based devices in voltage regulators are available, but they necessitate and separate additional die when used with high performance RF GaAs MMIC's. MMIC-integrated with GaAs HBT's will be more cost efficient and simplify the overall power module design and building process, while providing improved frequency performance. However, the temperature bias (or Vcc) sensitivity has limited this integrated use.
The present invention addresses these issues, making advantageous use of HBT transistors integrated in the power module. HBT devices can provide useful gain while operating at higher temperatures than traditional bipolar and/or CMOS devices. More-over, HBT devices can be made to provide useful gain at very high frequencies, e.g. up to six GHz or so.
It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to illustrative embodiments the drawings, and methods of use, the present invention is not intended to be limited to these embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be defined as only set forth in the accompanying claims.